Remove startupconfig.json support, support relative embedded app path in apphost #3103
Description
As a follow-up to dotnet/core-setup#3888, it was decided to remove the startupconfig.json support and instead support having the existing embedded dll name support a relative path instead. This makes it more trustworthy.
Currently, the dll name (which is embedded into the apphost at build time) can only be a dll name, so this will change that to allow a relative path plus the dll name.
The relative path is to use / for path delimiters, which will get converted to \ on Windows.